1. Field of the Invention
This invention generally relates to the field of medical equipment for respiratory therapy and more specifically to the field of equipment for monitoring the breathing characteristics of a human patient who is breathing with the assistance of a ventilator.
Clinical treatment of a ventilated patient often calls for monitoring a patient's breathing to detect an interruption or an irregularity in the breathing pattern, for triggering a ventilator to initiate assisted breathing, and for interrupting the assisted breathing periodically to wean the patient off of the assisted breathing regime, thereby restoring the patient's ability to breath independently.
2. Prior Art
Monitor systems for the detection of apnea, the medical term for interruption or cessation of normal breathing in a patient are known. For example U.S. Pat. No. 4,519,387 to Durkan et al discloses a control circuit operating a valve to supply respiratory gas to a person if the person's breath does not occur within a predetermined, yet selectively variable, interval. To detect a cessation of breath, earlier systems used a variety of techniques, including measuring the impedance through the patient, which varies with patient respiration, as in U.S Pat. No. 4,580,575 to Birnbaum.
U.S. Pat. No. 3,911,899 to Hatties teaches the use of an electromagnetic sensor comprising a transmitter and a receiver attached to a patient,s chest to detect breathing. The sensor generates a signal in response to the expansion and contraction of the patient's chest. The signals produced are detected and filtered to determine if a breath has occurred. The signals produced by the sensor are influenced by any motion of the chest, even motion not related to breathing. An adjustable timer is provided for the detection of an apnea episode. A flow rate signal is not compared with an adjustable flow-rate signal level to determine if a breath is a valid breath. An output signal is not provided to control a ventilator.
U.S. Pat. No. 4,365,636 to Barker teaches the measurement of respiration activity as a signal voltage obtained from sensors attached directly to the body. The signal is therefore, a measure of change of body position or volume as a function of time. The signal measured is scaled and sampled at a fixed sample rate of eight or more cycles per breath. The amplitude of the samples is processed to determine the slopes of the waveform at points on the waveform, the slope values being stored. The slope values thus calculated characterize the movement of the part of the patient's anatomy to which the sensor is affixed.
U.S. Pat. No. 4,414,982 to Durkan shows a pressure sensor for the detection of inspiration by sensing a negative pressure of as little as 0.5 millimeters of water.
None of the above references teaches, discloses or suggests a ventilator control system that senses inspiratory flow rate to determine if the sensed flow rate signal exceeds the amplitude of an operator selected threshold to qualify a patient's breath as a valid breath, and that allows the operator to select and adjust the duration within which a valid breath must occur to avoid signaling the ventilator to provide one or more mandatory breathing cycles.
The prior art systems have not provided a ventilator control system that permits an operator to selectively adjust an inspiratory flow rate threshold for the patient being monitored, which threshold, if not exceeded within a selectively adjustable interval, results in a triggering episode that automatically actuates a ventilator to deliver a selected number of breaths.